This invention relates to a method of manufacturing an optical fiber-preform, and more particularly to a method of manufacturing a preform for an asymmetrical optical fiber.
As used herein, the term "asymmetrical optical fiber" is defined to mean that type wherein a common glass cylindrical cladding contains a plural fibers prepared from the glass having different properties from the cladding glass; all the fiber act as core, or some of them function as stress-applying parts; all the glass fibers are integrally embedded in the cladding; and at least one of the plural glass fibers is positioned apart from the central axis of the cladding. In contrast, as shown in FIG. 1, the conventional single mode optical fiber contains only one core 20 extending along the central axis of cladding 10.
Description may now be made with reference to FIGS. 2A to 2C of an asymmetrical optical fiber FIG. 2A or 2B represents a first example of an asymmetrical optical fiber. Throughout the figures set forth, reference numeral 10 denotes a cladding. Two cores 20 are lengthwise embedded in cladding 10. Core 20 is formed of the glass having a larger refractive index than that of cladding 10 so as to cause light beams transmitted through core 20 to perform a substantially total reflection at an interface between cladding 10 and core 20. Two cores are positioned apart from the central axis of cladding 10.
Referring to the above-mentioned twin-core type optical fiber, the type of FIG. 2A wherein two cores 20 are as spaced from each other as to prevent light beams passing therethrough from interfering each other can be applied as a sensor, if two cores 20 are made to have different properties. When external environmental factors such as atmospheric temperature and pressure are applied to the optical fiber, changes appear in the state of light transmitting through cores 20. If cores 20 are let to have different properties in advance, it is possible to detect the magnitude of received external environmental factors from the difference between the state of light in one core and that in the other core.
If two cores 20 are positioned near to each other as shown in FIG. 2B, then light transmitted therethrough can be coupled together. Therefore, the twin-core type optical fiber can be applied as a coupler or isolator.
A second example of asymmetrical optical fiber shown in FIG. 2C is referred to as "a polarization-maintaining optical fiber." In the second example of FIG. 2C, only one core 20 extends along the central axis of cladding 10. Two stress-applying parts 30 are embedded lengthwise in cladding 10. Stress-applying parts 30 are prepared from glass material having a larger thermal expansion coefficient than cladding 10.
The above-mentioned polarization-maintaining optical fiber is characterized in that even when light is transmitted while the fibers are warped, the polarization at the input end can be sustained even though light is transmitted through a long distance. Therefore, said polarization-maintaining optical fiber can be applied in a wide field including a sensor like a fiber gyroscope and coherent optical communication based on only a particular polarization of light.
In obtaining an asymmetrical optical fiber, a preform which is previously manufactured by the method described below is elongated to have the predetermined diameter.
The conventional method of manufacturing a preform for an asymmetrical optical fiber involves the rod-in-tube method or pit-in-tube method. Description may now be made of these prior art methods.
As illustrated in FIG. 3A, cladding-mother rod 11, core-mother rod 21, stress-applying-mother rods 31 are provided in advance Mother rods 11, 21, 31 can be produced by the widely known processes such as VAD (vapor-phase axial deposition), OVPO (outside vapor phase oxidation) and MCVD (modified chemical vapor deposition).
Thereafter, holes 14 for insertion of core-mother rod 21 and stress-applying-mother rod 31 are perforated in cladding-mother rod 11 by means of drill 40. Each hole 14 is mentioned as "drilled-pore" hereinafter. Later as shown in FIG. 3C, core-mother rod 21 and stress-applying-mother rod 31 are inserted into corresponding drilled pore 14.
Later as indicated in FIG. 3D, heating is externally applied to cladding-mother rod 11 by means of, for example, flames 44 of burner 42. Thus, the boundaries between the inserted mother rods 21, 31 and cladding-mother rod 11 are fused together, thereby providing a perfectly integrated transparent preform 90.
The preform for the twin-core type optical fiber shown in FIGS. 2A and 2B is fabricated in the same manner as mentioned above.
The conventional rod-in-tube process and pit-in-tube process are accompanied with the under mentioned difficulties. First, limitation has to be imposed on the size of a preform to be obtained. If it is attempted to obtain a long preform, a necessarily long drilled-pore 14 will have to be perforated in cladding-mother rod 11. However, any present technique cannot perforate such a long drilled pore through a glass rod, thus imposing a certain limitation on the length of a preform to be fabricated.
Secondly, impurity contamination or scratches tend to appear on the inner wall of drilled pore 14 or the surface of core-mother rod 21 and stress-applying mother rod 31. Therefore, when the preform is drawn to provide an optical fiber, an impurity may diffuse into the core or bubbles will appear in the fiber. These events lead to the transmission loss of light passing through the core.